The LHR Stasi should have been as pro-active with guarding this fence as they are with hassling and being unpleasant to innocent members of the public.
Judging by the reports the fence involved was at east base, not on the airport proper..
I can only presume the fire brigade used cutting equipment to get him out
I rather suspect your assumption is incorrect. There have been few hints in previous posts as to how access could have been obtained without cutting anything.